A plant that feeds on insects is not immune from being eaten by other insects. Any moth that drinks nectar from the leaf without falling into the trap, or any beetle that eats pollen without carrying some
The sweet pitcher plant, Sarracenia rubra, takes its common name from the fragrance of its red flowers. It grows in four separate locations: along the southeastern coastal plain, on the Gulf coastal plain, in a few bogs in the Carolina mountains, and in another small area in the middle of Alabama. The typical pitchers are small, 6 inches to 12 inches, but the mountain populations and some of the Gulf plants grow to 27 inches tall.
away on its sleek body to fertilize another flower, reduces the resources of the plant. But these are only chance visitors, and by chance they succeed in taking something from the plant without serving its needs. The lives of other insects are completely intertwined with the Sarracenia plants.
Small green Tortrix caterpillars feed on the petals and other flower parts. When fully grown, the caterpillars spin a loose cocoon around the wreckage of the flower. They stay there through the pupal stage and then emerge as small, dark moths.
All species of Sarracenia have thick underground stems, rhizomes, filled with starches made by the plant. The rhizomes continue to grow for twenty or
Caterpillar tunneling within a pitcher plant rhizome thirty years and may become one foot long. One pitcher insect lives by raiding this plump storehouse. In its caterpillar stage, the insect enters a rhizome by boring in through a leaf bud. Then it eats down the length of the rhizome, making a passage through it. Waste from its tunneling is brought back to the entrance and piled up there, forming a tube that stands upright around the hole. These tubes are raised to two inches or higher by the growing larva. As the summer wears on, the caterpillar digs a special exit before it pupates in a wide part of the tunnel. The handsome moth that emerges wears the colors of the pitcher plants, maroon and yellow. It crawls from the exit hole in the fall, and the female lays seedlike
Pupa inside a rhizome eggs. When they hatch in the spring, the young caterpillars eat their way into another pitcher rhizome to repeat the cycle.
Other animals invade even the leaves and turn these death traps to their own advantage. Again, some are only chance visitors. They have no necessary connection with these plants but, having stumbled upon the pitchers, they use the opportunity. Slugs and snails that lurk about the mouths of pitchers belong to this group, as well as the various spiders that spread nets across the tube and drop on silken life lines to snatch prey from the pitcher's pool. Small frogs are occasional invaders. The shadowed cranny of the pitcher, with its changing parade of visiting insects, offers obvious attractions to all of these.
None of these animals is entirely safe from the danger of the trap, and some of them never leave the leaf; but a number of insects spend a large part of their lives within the pitcher leaves without suffering any harm. They depend upon the pitchers and live only where these plants grow. Their lives are adjusted to the complicated design of the pitcher leaf: Their cycles of development and the workings of their small bodies are fine-tuned to the structure of the pitcher leaf and to the chemicals that pour into it. They live there without danger and draw upon the leaf for their own life needs.
The pitcher plant mosquito enters newly opened leaves of the purple pitcher to deposit eggs on the pool of liquid or the moist inner surface of the leaves. The eggs hatch and the mosquito larvae, or wrigglers, swim out into the pitcher fluid. The wrigglers live in the leaf's well until they are ready to fly off as adults. They take their food from the pitcher liquid, swimming freely among the dead prey. In the northern part of this plant's range where the water inside the pitcher becomes a core of ice in winter, some of the larvae spend their hibernation frozen within the dead leaf.
Purple pitcher leaves produce only weak digestive enzymes, and the fluid is always diluted by rain that pours into their uncovered mouths. Still, the same liquid that digests captured insects seems to have no effect on these tender, soft-bodied wrigglers.
Several species of flesh flies also live in pitcher leaves during the early part of their lives and plunder the pitcher's catch of insects. The parent fly places small maggots on the upper part of the pitcher interior. These young fall into the pitcher liquid and begin to feed on the carrion mass. They are not affected by the plant's juices, because their bodies produce a substance that protects them from the action of the pitcher's digestive enzymes. They eat and thrive, squirming up through the growing pile of dead bodies. When full size, about three-fourths of an inch long, the big white maggots eat a hole in the pitcher wall and leave it to pupate in the ground.
Of all the insects that leave their young to grow within the pitchers, a solitary wasp makes the most elaborate preparations. Wherever pitchers grow within the range of this southern insect, it uses them as protective shelters for its eggs. The Sarracenia wasp stuffs the lower part of a pitcher tube with a plug of grass or moss. It then packs the pitcher with layers of freshly killed grasshoppers or crickets, separated by more loose wads of grass. The wasp lays eggs among the dead insects and then tops everything off with another wad of plucked grass or moss. The young develop within the sealed leaf, the dead insects providing a generous supply of food.
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